L1-dependent neuritogenesis involves ankyrinB that mediates L1-CAM coupling with retrograde actin flow

T he cell adhesion molecule L1 (L1-CAM) plays critical roles in neurite growth. Its cytoplasmic domain (L1 CD) binds to ankyrins that associate with the spectrin-actin network. This paper demonstrates that L1-CAM interactions with ankyrin(B) (but not with ankyrin(G)) are involved in the initial formation of neurites. In the membranous protrusions surrounding the soma before neuritogenesis, filamentous actin (F-actin) and ankyrin(B) continuously move toward the soma (retrograde flow). Bead-tracking experiments show that ankyrinB mediates L1-CAM coupling with retrograde F-actin flow in these perisomatic structures. Ligation of the L1-CAM ectodomain by an immobile substrate induces L1CD-ankyrin(B) binding and the formation of stationary ankyrin(B) clusters. Neurite initiation preferentially occurs at the site of these clusters. In contrast, ankyrin(B) is involved neither in L1-CAM coupling with F-actin flow in growth cones nor in L1-based neurite elongation. Our results indicate that ankyrin(B) promotes neurite initiation by acting as a component of the clutch module that transmits traction force generated by F-actin flow to the extracellular substrate via L1-CAM.